1. Field of the Invention
The present invention is directed to a two-wire full-duplex modem which time-divides data into main channel data and secondary channel data by using a two-wire line in sending and receiving data.
In the market for modems for private lines, to meet a demand for reduction in line costs, two-wire full-duplex modems are gradually replacing four-wire full-duplex modems.
If a modem is the two-wire type, the charge for four wires is not applied, and therefore, a reduction in the line charge of about 5,000 yen per month is possible. In order to deal with a fault on private line in which two-wire full-duplex modems are used, the demand for automatic backup has increased, and so has the demand for network supervision.
2. Description of the Related Art
In two-wire full-duplex 9,600 bps modems, for example, a time division circuit time-divides 12,000 bps data which are in conformity with the CCITT recommendation V. 32 bis into main channel data of 9,600 bps and secondary channel data of 2,400 bps. The data are then scrambled and modulated by a transmitting means, and sent to a private line through a 2 W-4 W conversion circuit.
On the other hand, since a receive signal from the 2 W-4 W conversion circuit includes an echo component of a send signal, in full-duplex communication, an echo estimate part predicts the echo component of the transmitted signal. The echo component is subtracted by an adder part from the receive signal which is produced by the 2 W-4 W conversion circuit. Then, the signal is demodulated and descrambled by a receiving part, and divided into a 9,600 bps main channel and 2,400 bps secondary channel before being sent out to the terminal side.
In this manner, two-wire full-duplex communication is achieved.
By the way, the function of the secondary channel is to transmit a supervisory signal and a control signal to the network. These are low-speed signals of around 75 bps. Hence, the speed of these slow signals must be increased to the assigned speed of 2,400 bps. To this end, a received 75 bps signal is sampled by multipoint sampling, and thereby a 2,400 bps output signal is obtained.
When there is trouble on the private line, modems of this construction switch to a public line (i.e., a backup line).
However, modems of conventional construction have the following drawbacks:
(1) Even when a private line is disconnected, in a two-wire full-duplex modem, since the 2 W-4 W conversion circuit is connected to the line, the impedance of the line changes and becomes high. Hence, due to a change in the impedance, the echo component of the send signal changes and therefore remains even after subtraction of the echo component by the echo estimate part. The echo component will therefore appear as a receive signal and will be detected as a carrier by the receive part. As a result, an automatic equalizer operates, thereby performing normal receiving operation. Thus, the disconnection of the line cannot be detected, which makes it difficult to switch to the backup system.
(2) Similarly, since the quality of communication of the secondary channel is the same as the quality of communication of the main channel, when the line deteriorates, supervisory control by means of the secondary channel will be obstructed. This makes it difficult to switch to the backup system.